Aeroacoustic noise generation due to vortex reconnection

We address our long-standing claim that vortex reconnection is one of the dominant sources of aeroacoustic noise in a number of canonical turbulent flows. The reconnection of two antiparallel vortices is studied via direct numerical simulation of the compressible Navier-Stokes equations in order to fully resolve the acoustic noise generation and far-field sound wave propagation. We show that the primary acoustic source is initially located at the contact point and then at the bridges, where reconnected vortex lines accumulate. At the start of reconnection, the sharp near-field pressure rise results in a high level of far-field noise with a clear dipole pattern. As more vortex lines reconnect and recoil from each other by self-induction, the near-field low-pressure zone extends in both the axial and lateral directions, which results in a quadrupole far-field noise. We thus reveal and quantify sound pressure levels and directivity during vortex reconnection. This work paves the way for further investigations of the reconnection generated noise, especially at higher Reynolds numbers.